The subject of the present invention is a block of juxtaposed sheets and a water-retention reservoir structure formed by a set of these blocks and intended for various uses, for example as a buried tank for retaining rainwater during very heavy rain, this structure then being connected to a water inlet and outlet collector system.
As is known, the continuous development of urban conglomerations results in an increase in areas of impermeable ground. In particular, car parks (for shopping centres, airports, leisure parks, etc.) and urban roadways are generally impermeable areas or nearly so, from which rainwater rapidly runs off towards the drainage systems. This impermeability prevents direct infiltration of rainwater. The result is that, during very heavy downpours, such as those which happen during storms, collectors designed for relatively low flow rates may be saturated and flooding in urban areas may occur.
Another consequence of this overloading of the water drainage systems (case of combined sewerage) is the direct discharge of polluted water into the receiving watercourse through the storm-water overflows, which may have particularly disastrous effects on the ecosystem.
When the inadequacy is limited to certain sections of the system, reinforcements are possible, but when it is the entire system which approaches saturation, it is necessary to find novel solutions, the reinforcement of one section possibly leading to a downstream spill-over. In order to face up to these problems, it is necessary either to delay considerably the flow-away (several hours being sometimes necessary), or to prevent any flow (by infiltration of the rainwater, for example), or even to combine these two solutions. This is possible by means of a temporary retention of the rainwater by retention tanks in the form of underground reservoirs.
Constructions, such as car parks, roadways, sports grounds, etc., integrating the storage of rainwater in a porous material into their structure, form part of these solutions and are called "reservoir structures". In this case, the porous material possesses at least two, hydraulic and mechanical, functions, and is generally located beneath the level of the natural terrain. This enables the surface water to be easily collected therein.
A buried water-retention reservoir structure is thus known from WO-A-88/00422, in which the upper part is constituted by a structure having mutually parallel vertical cells, resting on a lower part for horizontal drainage of the water, this lower part being formed, for example, by a gravel/sand mixture or by a porous material.
Such a reservoir structure takes a long time to install and furthermore requires heavy implementation means, so that it is very expensive to realize. In addition, its lower drainage structure may, after a certain time, be blocked by the accumulation of sand entrained by the water, and thereby render the entire structure inoperational.